Do not inhibit idle in borgmatic.service

When backing up a machine with a monitor using logind to control idle
timeout and things like DPMS, borgmatic can block the screen from
turning on/off with systemd-inhibit. This is because by default
systemd-inhibit will block "idle:sleep:shutdown". Borgmatic does not
need to care about idle, only about suspend and shutdown. So, add an
explicit `--what` flag for what borgmatic should inhibit.

For more information see systemd-inhibit(1).

sample/systemd/borgmatic.service

@@ -61,4 +61,4 @@ LogRateLimitIntervalSec=0
 # Delay start to prevent backups running during boot. Note that systemd-inhibit requires dbus and
 # dbus-user-session to be installed.
 ExecStartPre=sleep 1m
-ExecStart=systemd-inhibit --who="borgmatic" --why="Prevent interrupting scheduled backup" /root/.local/bin/borgmatic --verbosity -1 --syslog-verbosity 1
+ExecStart=systemd-inhibit --who="borgmatic" --what="sleep:shutdown" --why="Prevent interrupting scheduled backup" /root/.local/bin/borgmatic --verbosity -1 --syslog-verbosity 1